This invention relates to a compact zoom lens system having a zoom ratio of about 1.7 and so lending itself well fit for lens shutter cameras, etc., and more particularly to a zoom lens system comprising three units, say, positive, positive and negative units.
In recent years, zoom lenses smaller in size and lighter in weight have been increasingly demanded to keep pace with lens shutter cameras having a more increased zooming function. A size reduction of zoom lenses for recent lens shutter cameras is tantamount to decreasing mainly the axial length of the lens system (the length from the first surface of the lens system to the film plane). In most cases, this is now achieved by retracting the barrel into the body of a camera when it is kept. However, it is then required to use some barrel-retracting mechanism, which sets limits to the size reduction of a camera body; in other words, the size reduction of a camera should be achieved by reducing the diameters of the lenses or making the optical system short.
So far, some three-unit zoom lenses have been proposed, which comprise a first lens unit having a positive focal length, a second lens unit having a positive focal length and a third lens unit having a negative focal length. Among them, zoom lenses having a zoom ratio of about 1.5 are disclosed in Japanese Provisional Patent Publications Nos. Sho. 60-263113, 61-52620 and 61-109012 and those having a zoom ratio of about 2.0 are set forth in Japanese Provisional Patent Publication No. Hei. 1-93713. These are all made compact in the axial direction while maintaining the power of each unit properly, but they are not in the diametrical direction of the lenses, because the diameter of the image-side lens is increased due to their short back focus. In addition, their short back focus gives rise to a reduction in their distance from the final surface of the lens system to the film plane; flaring is much likely to occur because the light reflected off the image plane is again reflected from the final surface.